3 Watt FM Transmitter

This is the schematic for an FM transmitter with 3 to 3.5 W
output power that can be used between 90 and 110 MHz. Although the stability isn't so bad, a PLL can be used on this circuit.

This is a circuit that I've build a few years ago for a friend, who used it
in combination with the BLY88 amplifier to obtain 20 W output power. From
the notes that I made at the original schematic, it worked fine with a SWR
of 1 : 1.05 (quite normal at my place with my antenna).

Schematic

Parts

Part

Total Qty.

Description

Substitutions

R1,R4,R14,R15

4

10K 1/4W Resistor

R2,R3

2

22K 1/4W Resistor

R5,R13

2

3.9K 1/4W Resistor

R6,R11

2

680 Ohm 1/4W Resistor

R7

1

150 Ohm 1/4W Resistor

R8,R12

2

100 Ohm 1/4W Resistor

R9

1

68 Ohm 1/4W Resistor

R10

1

6.8K 1/4W Resistor

C1

1

4.7pF Ceramic Disc Capacitor

C2,C3,C4,C5,C7,C11,C12

7

100nF Ceramic Disc Capacitor

C6,C9,C10

3

10nF Ceramic Disc Capacitor

C8,C14

2

60pF Trimmer Capacitor

C13

1

82pF Ceramic Disc Capacitor

C15

1

27pF Ceramic Disc Capacitor

C16

1

22pF Ceramic Disc Capacitor

C17

1

10uF 25V Electrolytic Capacitor

C18

1

33pF Ceramic Disc Capacitor

C19

1

18pF Ceramic Disc Capacitor

C20

1

12pF Ceramic Disc Capacitor

C21,C22,C23,C24

4

40pF Trimmer Capacitor

C25

1

5pF Ceramic Disc Capacitor

L1

1

5 WDG, Dia 6 mm, 1 mm CuAg, Space 1 mm

L2,L3,L5,L7,L9

5

6-hole Ferroxcube Wide band HF Choke (5 WDG)

L4,L6,L8

3

1.5 WDG, Dia 6 mm, 1 mm CuAg, Space 1 mm

L10

1

8 WDG, Dia 5 mm, 1 mm CuAg, Space 1 mm

D1

1

BB405

BB102 or equal (most varicaps with C = 2-20 pF [approx.] will do)

Q1

1

2N3866

Q2,Q4

2

2N2219A

Q3

1

BF115

Q5

1

2N3553

U1

1

7810 Regulator

MIC

1

Electret Microphone

MISC

1

PC Board, Wire For Antenna, Heatsinks

Notes

The circuit has been tested on a normal RF-testing breadboard (with one side copper). Make some connections between the two sides. Build the transmitter in a RF-proof casing, use good connectors and cable, make a shielding between the different stages, and be aware of all the other RF rules of building.

Q1 and Q5 should be cooled with a heat sink. The case-pin of Q4 should be grounded.

C24 is for the frequency adjustment. The other trimmers must be adjusted to maximum output power with minimum SWR and input current.

Local laws in some states, provinces or countries may prohibit the operation of this transmitter. Check with the local authorities.

Comments

hello sir how r u? thanks for fm circuit. plz tell me more about L2,L3,L5,L7,L9 6-hole Ferroxcube Wide band HF Choke (5 WDG) is it available in market? if yes then whats its name plz tell me in details sir thanks i m waiting ur answer sir thanks again

What the hell is wrong with all those lazy people? Why can't they just try putting together the circuits themselves instead of having to be spoon-fed the whole building process. Even weirder is, that some of them are claiming to be studying electronics related subjects and needing these circuits for their graduation project. If that's true, than how can't you understand these fairly simple diagrams, and what does that say about the intelligence of students or colleges/universities? It's mind boggling when you think about it.

Stop begging please. This is an easy schematic, so it shouldn't take you long to create a pcb layout from it. If you can't then you shouldn't start with this project. If you don't know how to make the coils/inductors, go start with a simple transmitter, google for "FM BUG". With these small transmitters yo ucan gain a lot of experience, then you can start with bigger projects.
Stop asking about teh range, because the range depends on many factors, how you build the transmitter, your swr, and the quality of the coax etc.
All passive components are available on the market, except for the 2N3866 and 2N3553 but you still can buy them from e-bay (watch out for fakes), well some companies still make them but you'll need to find it out yourself, I'm not going to post any links here. If you want any other transistors you need to checkout the datasheets of them and compare them with available transistors. For BF115 you can use a BF199.
7810 is a voltage regulator, you can buy this component it's really cheap (yes even for Indians). Look for 7810 or 78L10, they're available in a TO-92 and a TO-220 package.
And people don't forget to use a dummyload when you're configurating the stages and power. If you don't use a dummyload you will burnout the final transistor.
For the stability, you need to cool your transistors with good heatsinks, use the proper shielding for every stage, and build a good metal case around your transmitters pcb.
The ferroxcube thingy is a simple wide band choke with 6 holes in it. If you don't know what it is google it (image's), you can get this part everywhere.
And my final words are; don't use this crappy transmitter for serious businness! Because it will interfere on the whole RF band, it will jam important frequencies (air traffic, emergency etc)!
I will build this transmitter this month with a fifteen watt amplifier. Imma going to check this page often.

this transmitter with a home made broadcast centre can do wonders.try it with multireplier and communication within 3 miles using a modulated mike in specific radios will do wonders.i made it why not you!

Well to those who are having trouble, it may help to a) try building an simple(r) FM transmitter (google search) or b) try building an AM transmitter, unless FM is necessary.
Also, looking up what each individual component is, and what it's job is in this project may also help.
(slightly simpler) ones (for casual learning/fun):
http://www.pira.cz/entx1.htm

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